There have been widely researched, developed, and commercialized information recording media that allow signals to be recorded thereon and erased therefrom by irradiating a thin film formed of, for example, a chalcogen material with a laser beam to heat it locally and causing a phase change between states with different optical constants due to the difference in irradiation conditions.
Generally, information is recorded by irradiating a recording layer with a high power (recording power) laser beam so as to melt the recording layer, and cooling it rapidly to turn the irradiated portion to an amorphous phase. In contrast, when erasing information, the recording layer is irradiated with a laser beam at a lower power (erasing power) than that used for recording so as to increase the temperature of the recording layer, and cooling it gradually to turn the irradiated portion to a crystalline phase. Thus, the information recorded before is erased. Hence, it is possible to record new information (rewrite) on phase-change optical information recording media while erasing the recorded information therefrom by allowing its recording layer to be irradiated with a laser beam at a power modulated between a high power level and a low power level.
More specifically, the phase-change information recording media allow information to be recorded thereon and reproduced therefrom by utilizing the phenomenon that the amounts of reflected light are different between the crystalline state and the amorphous state when using light with a specified wavelength. Thus, erasure and recording of information are performed at the same time by modulating the output power of the laser beam.
In recent years, various techniques have been studied for increasing the capacities of optical recording media. For example, there has been studied a technique for performing high-density recording, in which the spot diameter of the laser beam is reduced by using a blue-violet laser that has a shorter wavelength than that of a conventional red laser, and/or reducing the thickness of a substrate on a laser beam incident side and using an objective lens with a high numerical aperture (NA).
There already has been commercialized an information recording medium (see Patent Literature 1) including two information layers, in which information is recorded on and reproduced from the two information layers by using a laser beam that is incident from one side of the information recording medium.
For example, there has been commercialized a 120 mm-diameter BD-RE medium that allows information to be recorded thereon and reproduced therefrom using a laser beam at a wavelength of 405 nm, and has a recording capacity of 50 GB per one side (in total of two information layers).
In an optical information recording medium including two information layers on and from which information can be recorded and reproduced, recording and reproducing information on and from the information layer (a first information layer) disposed farther from the laser beam incident side are performed with a laser beam that has passed through the other information layer (a second information layer) located closer to the laser beam incident side.